When the top of the water surface is flat, gas will not exchange. You can have a lot of water flow down to the bottom of the tank, but it is almost useless because we`re trying to create agitation at the surface, which allows gas exchange. The most important thing to the fish besides the water is oxygen, thus we need to have plenty of oxygen in our aquariums. We are really looking for oxygen saturation and there are two ways that you can provide oxygen: either via an air pump, which is going to give you a nice stream of bubbles, or by using a water pump, which is going to break the water surface. Breaking the water surface is extremely important with tanks like cichlid tanks because of how heavily stocked they need to be. For instance, if you are going to have a 200lt aquarium, you`re probably going to put thirty, if not forty fish in it, and adequate gas exchange will ensure that each fish gets enough oxygen. So, putting in a nice, big air stone will be your best bet in providing plentiful agitation of the water surface.

The next thing to understand is that your filter does not aerate the water, because the water that goes out of the filter will be very low in oxygen. One of the main things the filter does is a process called oxidization, which means it uses bacteria which, in turn, use oxygen to break down the waste of the fish. So, the oxidization is sucking all the oxygen out of the water. Therefore, when the water comes out of the filter, it is oxygen deficient, because the bacteria have just stripped all the oxygen out for the oxidization. If it agitates the surface while it is being returned to the tank, then it is a process of re-oxygenation, not oxygenation. You should still have a separate pump for aeration.

So, when you go home and check out your aquarium, make sure that you have something to aerate and something to filter. And remember that they`re not the same.

As per.com Paul Talbot
Taken from www.cichlidhub.com